Countertop Calculator
Calculate countertop square footage from the counter run, depth, island and backsplash, then see how many quartz, granite or laminate slabs you need and the cost.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Countertop area
- 49.2 sq ft
- 42.5 sq ft of wall counter, 6.7 sq ft of backsplash
- Area with waste
- 56.5 sq ft
- 15% added for seams, cutouts and slab layout
- Slabs needed
- 2 slabs
- 3 cm slabs of 55 x 120 in, 45.8 sq ft each. At least 1 seam; ask for the layout drawing before cutting
- Finished edge
- 20 ft
- Front edge of the wall counters; edge profiles are priced per linear foot
Countertop formula
Counter area (sq ft) = Counter run (ft) x Depth (in) / 12
Island area (sq ft) = Island length (ft) x Island width (in) / 12
Backsplash area (sq ft) = Counter run (ft) x Backsplash height (in) / 12
Slabs = (Counter + Island + Backsplash) x (1 + waste %) / Slab area, rounded upWorked example
A kitchen with 14 ft of wall counters 25.5 in deep, a 6 ft x 42 in island and no backsplash, in jumbo quartz with 20% waste: 14 x 25.5 / 12 = 29.75 sq ft of wall counter. Island: 6 x 42 / 12 = 21 sq ft. Total 50.75 sq ft. With 20% waste: 50.75 x 1.2 = 60.9 sq ft. Jumbo slabs are 58.7 sq ft, so 60.9 / 58.7 = 1.04, which means 2 slabs.
Finished edge: 14 ft along the front of the wall counters plus 2 x (6 + 3.5) = 19 ft around the island, for 33 ft of edge profile. At $70 per sq ft installed, 60.9 x 70 = $4,263.
Quick reference
| Counter run | Countertop area | With 15% waste | 3 cm slabs (55 x 120 in) |
|---|---|---|---|
| 10 ft | 24.6 sq ft | 28.3 sq ft | 1 |
| 15 ft | 36.9 sq ft | 42.4 sq ft | 1 |
| 20 ft | 49.2 sq ft | 56.5 sq ft | 2 |
| 25 ft | 61.5 sq ft | 70.7 sq ft | 2 |
| 30 ft | 73.8 sq ft | 84.8 sq ft | 2 |
| 40 ft | 98.3 sq ft | 113.1 sq ft | 3 |
How to measure for countertops
Measure each cabinet run along the back wall, in inches, and sketch the kitchen with every dimension. Multiply each run by the finished depth, normally 25.5 in, and convert to square feet by dividing by 144. In an L or U shape measure the long leg full length and the short leg from the corner outward so the corner is not counted twice. Add the island as its own rectangle, including any 12 in seating overhang. Add a 4 in backsplash along the wall runs if you want a matching splash.
Sink and cooktop cutouts are not subtracted. The fabricator cuts them from the slab and charges for the full rectangle, plus a cutout fee, typically $150 to $300 for an undermount. Treat this number as a budget figure. The fabricator templates only after the cabinets are set, shimmed level within 1/8 in over 10 ft and screwed down, with the sink, cooktop and faucet on site. Install runs 1 to 3 weeks after template, and plumbing reconnect is usually a separate trade the next day.
Slabs, seams and waste
3 cm slabs run about 55 x 120 in and jumbo quartz up to 65 x 130 in. Any run over 10 ft (10 ft 10 in for jumbo) has a seam. Seams are 1/16 in joints of color-matched epoxy; solid-color quartz hides them and veined granite does not, so tag your actual slabs at the yard and ask for the layout drawing. A seam goes over a cabinet partition, never over the dishwasher opening and never at the corner of a sink cutout, where stone cracks. Whatever slab is left after layout is yours to pay for; 15 to 20% waste is typical, and an L or U with a wide sink wastes more.
A 10 ft by 25.5 in piece of 3 cm granite weighs about 380 lb and takes two or three people with a carry clamp; if it will not turn the corner into the kitchen, the fabricator adds a seam. Unsupported overhang tops out at 10 in on 3 cm granite (Natural Stone Institute) and 12 to 14 in on 3 cm quartz per the manufacturer. Past that, steel flat bar let into the cabinet tops or corbels carry the seating overhang, with at least two thirds of the top width bearing on cabinets. Laminate postform blanks come 4 to 12 ft with the 4 in splash built in; an L needs a factory miter pair pulled together with draw bolts, and no seam goes at the sink.
What this calculator leaves out
The sink and its cutout, cooktop cutout, faucet holes, edge profile upgrades by the linear foot, and any per-seam charge. Tear-out and disposal of the old top, typically $200 to $500 for stone, plumbing disconnect and reconnect, steel for overhangs, and a penetrating sealer on granite. Get the layout drawing and a line-item quote before paying the deposit.
Mistakes that cost money
- Templating before the sink, cooktop and faucet are on site adds a second trip fee and a week; have them at the house at template.
- A 12 in seating overhang on 3 cm granite with no steel cracks under a leaning adult; over 10 in gets flat bar or corbels.
- Cabinets out of level by more than 1/8 in over 10 ft crack stone at the seam; shim and screw them down before template day.
- Price per square foot rarely includes the sink cutout, edge upgrades, tear-out or plumbing reconnect; get each as a line item before signing.
- A laminate seam at the sink swells and lifts once water gets under it; seam over a cabinet partition and buy a factory miter.
- Picking granite from a 4 in sample: slabs vary, so tag the actual slabs at the yard and ask for the layout drawing before cutting.
Key facts
- A standard countertop is 25.5 in deep: 24 in base cabinets plus a 1.5 in front overhang.
- A typical 3 cm granite or quartz slab is about 55 x 120 in, or 45.8 sq ft; jumbo quartz slabs run 65 x 130 in, about 58.7 sq ft.
- Postform laminate sheets come 4 x 8, 5 x 12 and 4 x 12 ft; a 5 x 12 sheet is 60 sq ft.
- 3 cm (1.25 in) granite weighs about 18 lb per sq ft, so a 50 sq ft kitchen adds roughly 900 lb to the cabinets.
- Fabricators usually seam a run longer than the slab, so a 20 ft L-shaped kitchen from 10 ft slabs has at least one seam.
Frequently asked questions
Where will the seams be, and can I avoid them?
A 3 cm slab is 120 in long, so any run over 10 ft has a seam; jumbo quartz stretches that to 130 in. The fabricator puts it over a cabinet partition, away from the dishwasher opening and the sink corners, as a 1/16 in color-matched epoxy joint. Solid-color quartz hides a seam best; ask for the layout drawing before the slab is cut.
How many square feet is a slab of granite?
A standard 3 cm slab is about 55 x 120 in, or 45.8 sq ft, though slabs vary by quarry and some are 60 x 130 in. Usable area is less because of edges, cutouts and matching the pattern.
How much waste should I add for countertops?
Plan on 15% for a simple straight run and 20% or more for L and U shapes, islands and wide sink cutouts. The waste is slab material that cannot be laid out usefully, and you pay for the full slab.
Do I subtract the sink from countertop square footage?
No. Fabricators charge for the full rectangle because the sink and cooktop openings are cut from the slab after it is sized. Only a true void, such as a counter that ends at a range, is left out.
Sources and references
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